Granulosa cell tumors of the ovary have been identified in young SWR/J mice. Careful selection of a subline (designated SWR/Wt) has enhanced the incidence to 10 percent. Efforts to demonstrate viruses associated with these tumors have been unsuccessful. Studies are proposed to characterize the morphology and endocrinology of spontaneous and derived tumors; to further increase the incidence of the tumors; to continue preparation of recombinant inbred stocks for genetic analyses; and to begin examination of methods for treatment, and prevention of recurrences, of granulosa cell tumors.